The present invention relates to a carbon fiber product having balanced ultra-high Young's modulus and high tensile strength properties. More particularly, the invention pertains to carbon fibers having a modulus of greater than 100 Mpsi and a tensile strength of at least 500 kpsi derived from a solvent fractionated, mesophase pitch having a low quinoline insolubles content. The invention is also concerned with the process for preparing such ultra-high modulus and high tensile strength pitch carbon fibers.
In recent years, technical literature and patents are replete with disclosures of carbon fibers prepared from polyacrylonitrile and carbonaceous pitch. Processes utilizing polyacrylonitrile and processes utilizing a pitch fraction, i.e. mesophase pitch, as precursors are in commercial production. The use of mesophase pitch as the feed material would have numerous advantages, since polyacrylonitrile (PAN) fibers require expensive and complicated procedures, including the handling of toxic gas by-products.
High Young's modulus is one of the essential characteristics of commercial carbon fibers, since high stiffness is an important consideration when the carbon fibers are employed in the reinforcement of polymer, metal, and other matrices to provide advanced technology composites. Consequently, there has been considerable research on methods for enhancing modulus. Leonard S. Singer summarized the state of the prior art in his article entitled "Carbon Fibers from Mesophase Pitch" published in Fuel, Vol. 60 (1981, September) pp. 839-847.
While PAN based fibers provide high tensile strengths at low modulus (30 to 40 Mpsi), attaining high modulus PAN carbon fibers has proven difficult. For example, a 1986 review of current carbon fibers, reports the highest modulus PAN fiber has a tenacity of 355 kpsi at 71 Mpsi modulus. (J. D. H. Hughes, Carbon, Volume 24, page 551 (1986). It would be highly advantageous to produce carbon fibers that have high tensile strengths, i.e., greater than 500 kpsi, as well as ultra-high modulus. Such balanced properties, i.e. an excellent combination of ultra-high modulus and high tensile strength, would be highly desirable for a variety of commercial applications.
Previous investigations to improve the Young's modulus of carbon fibers derived from pitch have involved the use of heat treatment temperatures ranging from 1500.degree. to 3000.degree. C. Of course, graphitization increases at the higher temperatures.
Fischer and Ruland in "The Influence of Graphitization on the Mechanical Properties of Carbon Fibers", Colloid and Polymer Sciences, vol. 250, No. 8, pp. 917 to 920 (1980) reported that graphitization has an unfavorable effect on the mechanical properties, including tensile strength, of carbon fibers. Ng et al. in "Extended Abstracts of the 16th Biennial Conference on Carbon" Am. Chem. Soc., pp. 515-516 (1983) stated that high modulus carbon fibers spun from mesophase pitch performed less satisfactorily than PAN-based fiber.
A 1986 paper by Guigon and Oberlin, Composites Science and Technology, 25 (1986) pp. 231-241 reveals that the Young's modulus of mesophase pitch based carbon fibers increases with the average degree of graphitization. However, page 240 reports that the tensile strength is always low. The graphitization of polyacrylonitrile fibers also affects adversely the tensile strength. Published British Patent No. 2,170,491 (Pepper and Patton) discloses on page 1, lines 26-40, that the strength and modulus of these carbonized fibers increase rapidly up to about 1400.degree. C. Beyond 1400.degree. C., however, while the Young's modulus increases, tensile strength decreases, reportedly because the structure of the carbonized fibers becomes more representative of true graphite. As a result of these occurrences, the British Patent says that commercial PAN fibers are usually offered in a carbonized form with low modulus and high strength or in graphitized form with high modulus and low strength.
On the other hand, Published European Application 0159315 by Nakatani et al. emphasizes the need to balance the modulus of elasticity and tensile strength properties of carbon fibers based on acrylonitrile-type polymers. The inventors accomplish this by subjecting the fibers to a complex cycle of flame-resisting treatments, which include the application of elongation and then a series of carbonization treatments that do not exceed 1600.degree. C.
Recent investigations (e.g. U.S. Pat. No. 4,504,454--Riggs) to produce mesophase pitch-based carbon fibers with higher tensile strengths involve solvent fractionation treatment with the initial use of known organic solvents having a solubility parameter from 9.2 to 11 to separate insolubles, and then treating the solution with an organic solvent having a solubility parameter from 7.4 to 9.0 in order to recover insolubles that are convertible to the carbon fibers. Although increased tensile strengths were obtained, Young's moduli were substantially less than 100 Mpsi. The production of fibers with high modulus was not explored.
Another approach has been to employ special feed materials. One example is the synthetic compound described in U.S. Pat. No. 4,670,129 (Tate et al.). Another is to hydrogenate mixtures of coal tar or coal tar pitch and an aromatic oil, heat the hydrogenated product in the presence of a cracking catalyst, and there treat the soluble fraction of the resulting reaction product to form mesophase, as described in British Pat. No. 2,129,825. These are expensive processes.
Considerable effort to improve mechanical properties of msophase pitch based carbon fibers has involved modifying the shape of a conventional, circular cross-section spinneret so that it has an enlarged nozzle outlet and thereby produces a wavy cross-sectional fiber structure (Japanese published Patent Application 62-42320 Nakajima et al.--Kashima Oil). Earlier work employed spinnerets modified in such a manner that they give ellipsoidal or multilobal fibers with "leafy lamella" microstructure. Typical patents directed to such features are as follows:
Japanese Patent Application Publication 61-275426 Ohyabu et al. (Mitsui Coke KK).
U.S. Pat. No. 4,628,001, Sasaki et al. (Teijin Limited).
European Patent Application Publication 0219964, Edie et al. (Clemson University). Spinnerets described in these references have narrow slots and are difficult to manufacture and maintain.
In the March/April 1987 issue of the SAMPE Journal, pp. 27-31, David A. Schulz in "Advances in UHM Carbon Fibers: Production, Properties and Applications" concludes that the production of ultra-high modulus (UHM) carbon fibers from pitch is a complex process involving many operations and extreme conditions. According to the author, it is known that ultra-high modulus carbon fibers derived from mesophase pitch possess higher crystallinity and reach higher modulus levels than fibers made from other precursors. Table II of the article sets forth the properties of Amoco's UHM Thornel P-100 fibers. The grand average tensile strength was 356.4 kpsi and the grand average tensile modulus was 111 Mpsi, as measured by stand testing. Thus, despite many years of experience, Amoco's process has not led to improved tensile strength via the earlier patent disclosures.
It would be desirable, therefore, to have carbon fibers which exhibit balanced high tensile strength and ultra-high modulus. Moreover, it would be highly desirable to have the ability to produce such carbon fibers while avoiding the problems, special equipment, and special or exotic pitches required by the prior art in the manufacture of ultra-high modulus carbon fibers.